The invention discloses a foam aerated concrete block prepared from iron tailings at normal temperature and under normal pressure and a preparation method of the foam aerated concrete building block.The foamed aerated concrete block comprises the following components in percentage by weight: 20-50% of the iron tailings, 30-60% of cement, 2-4% of an early strength agent, 3-10% of fly ash, 1-3% ofcellulose, 1-3% of glue powder, 2-4% of polypropylene fibers, 1-2% of a foaming agent, 1-2% of a melamine water reducer, 1-2% of an organic silicon hydrophobic agent and 0.5-1.5% of a foam stabilizer.The foam concrete building block prepared by the invention has lower density, higher porosity, higher sound insulation property and stronger heat insulation property after being tested; the foam concrete building block prepared by the invention has excellent performance, and in a process of preparing the foam aerated concrete building block, process steps are simple, the reaction treatment time is extremely fast, the curing method is simple, and the production efficiency of enterprises can be greatly improved.